In the output circuit of display drive integrated circuits shown in FIG. 10, it has conventionally been advantageous to use the parasitic diode existing in a double diffusion MOSFET as diode D2, which is connected in parallel to an FET, if a double diffusion MOSFET (DMOS) is used as transistor N2 in the A region of this output circuit.
However, if an attempt is made to use a conductivity modulation MOSFET (IGBT) as transistor N2, diode D2 becomes inoperative because of the existence of a parasitic diode D4 serially connected to transistor N2 in addition to parasitic diode D2, as shown in FIG. 11. To make D2 operative, a parallel resistance R5 can be connected in parallel with diode D4 as shown in FIG. 12. A conductivity modulation MOSFET of anode short type that contains said type of circuitry is shown in FIG. 13.
In this conductivity modulation MOSFET, a p-type base region (23) and an n-type source region (24) are formed on the front surface of n-type conductivity modulation layer (22) by means of a double diffusion process, with an insulation layer (25), a gate electrode (28), and a source electrode (29) being disposed thereon. A minority carrier injection region (26) is formed by means of a diffusion process on the rear surface of the conductivity modulation layer (22). This rear surface is entirely covered by a drain electrode (27). In this case, the drain electrode (27) is structured so as to be in direct contact with the conductivity modulation layer (22), other than the minority carrier region (26), in order to shorten the time needed for the conductivity modulation MOSFET to transit to a low state of conductivity. Thus, a parallel circuit having parallel resistance R5 is formed. Parts D2 and D4, shown by dotted lines in FIG. 13, are parasitic diodes.
In this conductivity modulation MOSFET, when positive potential is applied to the gate electrode (28), electrons flow from the source region (24) to the conductivity modulation layer (22) via an inversion layer, resulting in holes to flow from the minority carrier injection region (26) into the conductivity modulation layer (22) as a result of the forward potential difference generated from a voltage drop in parallel resistance R5. This raises the conductivity of the conductivity modulation layer (22), and thus allows a large current to flow. Removal of the positive potential in the gate electrode (28) causes the inversion layer to disappear, the electrons to stop flowing in, the carrier to discharge, and the conductivity modulation layer (22) to become high in resistance again.
The circuit in region B of the circuit in FIG. 12 is called an "open-drain structure", and the circuit in region B can also be structured by a conductivity modulation MISFET of anode short type. If a conductivity modulation MISFET is used in a circuit of this type, a bonding pad or bump electrode is formed as an external connecting electrode at terminal DO in the figure, a multi-output drive circuit can be made that contains a large number of open-drain circuits (80 circuits, for example), and an equal number of DO terminals. Furthermore, each of the DO terminals is connected with a bonding pad or bump electrode.
In the above conductivity modulation MISFET of anode short type, conductivity modulation may become impossible if the value for parallel resistance R5 is too small. On the other hand, the effect of parallel resistance R5 cannot be obtained if the value for parallel resistance R5 is too large. Therefore, the resistance value of this parallel resistance R5 must be set at an adequate value and can be adjusted by changing the depth of the minority carrier injection region (26). However, if a diffusion process is used, diffusion to a great depth may reduce the accuracy of the shape of the diffusion region, thus making it difficult to obtain an accurate resistance value for controlling the minority carrier injection region (26) in the depth direction.
In addition, when this conductivity modulation MISFET of anode short type is incorporated in an integrated circuit, the element current is generally reduced, as is the voltage drop of parallel resistance R5. Therefore, to ensure an operation that will induce a conductivity modulation by applying a forward bias voltage (.about.0.7 V) between the minority carrier injection layer and the conductivity modulation layer, the value of parallel resistance R5 must be increased. However, increasing the resistance value is difficult because the setting range of the resistance value is restricted by the resistance factor of the conductivity modulation layer (22) and the element size. Thus separate resistance layers were heretofore required.
Furthermore, the existence of the drain electrode on the rear surface made it difficult to form an integrated circuit. It also became difficult to make the element separation technology and made the wiring arrangement more complex.
Moreover, connecting the drain electrode to a junction pad or bump electrodes on a large number of DO terminals causes many wirings to cross the elements. The wiring potential affects the elements, possibly causing a reduction in the breakdown voltage. In addition, the inability to form elements beneath the bonding pad or bump electrodes, in order to improve reliability, prevented a higher integration of the circuit.